The concept of a calcium sensor in transmitter release

Serum Concentration of Macro-, Micro-, and Trace Elements in Silver Fox (Vulpes vulpes) and Their Interrelationships with Morphometric, Densitometric, and Mechanical Properties of the Mandible

The optimal content of macro-, micro-, and trace elements in tissues ensures proper systemic growth and development and optimal health status in animals and humans. However, very little is known on the elemental content in the plasma compartment in Silver fox. The aim of this study was to determine the content of selected elements in serum obtained from 8-month-old female (N = 8) and male (N = 7) silver foxes. Moreover, relationships of the evaluated elements with the morphological, densitometric, and mechanical parameters of the mandible were determined. Serum content of 12 different elements was measured using inductively coupled plasma-atomic emission spectrometry. The morphometric and densitometric properties of the mandible were determined using quantitative computed tomography method, while mechanical endurance was tested using a three-point bending test. Serum concentration of calcium was significantly higher by 20% in male foxes (P = 0.01), while manganese concentration was significantly lower in males by over 17% (P = 0.03). Positive correlations of serum concentration of calcium, phosphorus, and magnesium with the morphological traits of the mandible such as weight, length, and bone volume were stated (P < 0.05). In the group of elements playing regulatory functions, the positive relationships between serum concentrations of selenium, chromium, manganese, copper, and cobalt were found (P < 0.05). The elaborated experimental model may serve for further studies on foxes, especially focused on nutritional factors affecting elemental homeostasis, whole-body metabolism, and systemic growth and development. Daily diet formulation and precise delivery for farm foxes, together with relatively large animal population maintained at the same environmental conditions, regularly subjected to slaughter procedure, enable economical experimentation with various dietary and pharmacological manipulations.

Allopregnanolone and puberty: modulatory effect on glutamate and GABA release and expression of 3α-hydroxysteroid oxidoreductase in the hypothalamus of female rats

The hypothalamic release of glutamate and GABA regulates neurosecretory functions that may control the onset of puberty. This release may be influenced by neurosteroids such as allopregnanolone. Using superfusion experiments we examined the role of allopregnanolone on the K(+)-evoked and basal [(3)H]-glutamate and [(3)H]-GABA release from mediobasal hypothalamus and anterior preoptic area in prepubertal, vaginal opening and pubertal (P) rats and evaluated its modulatory effect on GABAA and NMDA (N-methyl-d-aspartic acid) receptors. Also, we examined the hypothalamic activity and mRNA expression of 3α-hydroxysteroid oxidoreductase (3α-HSOR) - enzyme that synthesizes allopregnanolone - using a spectrophotometric method and RT-PCR, respectively. Allopregnanolone increased both the K(+)-evoked [(3)H]-glutamate and [(3)H]-GABA release in P rats, being the former effect mediated by the modulation of NMDA receptors - as was reverted by Mg(2+) and by the NMDA receptor antagonist AP-7 and the latter by the modulation of NMDA and GABAA receptors - as was reverted by Mg(2+) and the GABAA receptor antagonist bicuculline. The neurosteroid also increased the basal release of [(3)H]-glutamate in VO rats in an effect that was dependent on the modulation of NMDA receptors as was reverted by Mg(2+). On the other hand we show that allopregnanolone reduced the basal release of [(3)H]-GABA in P rats although we cannot elucidate the precise mechanism by which the neurosteroid exerted this latter effect. The enzymatic activity and the mRNA expression of 3α-HSOR were both increased in P rats regarding the other two studied stages of sexual development. These results suggest an important physiological function of allopregnanolone in the hypothalamus of the P rat where it might be involved in the 'fine tuning' of neurosecretory functions related to the biology of reproduction of the female rats.

New insights into molecular players involved in neurotransmitter release

The strength of a synapse can profoundly influence network function. How this strength is set at the molecular level is a key question in neuroscience. Here, we review a simple model of neurotransmission that serves as a convenient framework to discuss recent studies on RIM and synaptotagmin.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

A model for a G-protein-mediated mechanism for synaptic channel modulation

Neurons communicate with other neurons via specialized structures called synapses, at which the digital voltage signal encoded in an action potential is converted into an analog chemical signal. An action potential that arrives at the presynaptic face triggers release of neurotransmitter from vesicles in a calcium-dependent manner, and the neurotransmitter diffuses across the synaptic cleft and binds to receptors on the post-synaptic face, where it may trigger a postsynaptic action potential. Calcium is a critical component of the release process, and its spatio-temporal dynamics can control the release and can lead to facilitation or augmentation. However, how cells regulate cytoplasmic calcium so that exocytosis can be triggered successfully is still not completely understood. We propose a mechanism, based upon the experimental findings of Barrett and Rittenhouse [C.F. Barrett, A.R. Rittenhouse, Modulation of N-type calcium channel activity by G-proteins and protein kinase C, J. Gen. Physiol. 115 (3) (2000) 277], for the regulation of calcium influx through N-type channels in the presynaptic terminal by PKC and downstream effectors of G-protein activation. This proposed modulatory mechanism consists of a feedback loop involving cytoplasmic calcium, neurotransmitters and G-protein-coupled receptors. We study the dynamics of each component separately and then we address how kinetic properties of the components and the frequency of the stimuli affect the regulatory mechanisms presented here.

Physiology and Pathophysiology of the Calcium Store in the Endoplasmic Reticulum of Neurons

The endoplasmic reticulum (ER) is the largest single intracellular organelle, which is present in all types of nerve cells. The ER is an interconnected, internally continuous system of tubules and cisterns, which extends from the nuclear envelope to axons and presynaptic terminals, as well as to dendrites and dendritic spines. Ca2+release channels and Ca2+pumps residing in the ER membrane provide for its excitability. Regulated ER Ca2+release controls many neuronal functions, from plasmalemmal excitability to synaptic plasticity. Enzymatic cascades dependent on the Ca2+concentration in the ER lumen integrate rapid Ca2+signaling with long-lasting adaptive responses through modifications in protein synthesis and processing. Disruptions of ER Ca2+homeostasis are critically involved in various forms of neuropathology.

Single molecule observation of liposome-bilayer fusion thermally induced by soluble N-ethyl maleimide sensitive-factor attachment protein receptors (SNAREs)

A single molecule fluorescence assay is presented for studying the mechanism of soluble N-ethyl maleimide sensitive-factor attachment protein receptors (SNAREs)-mediated liposome fusion to supported lipid bilayers. The three neuronal SNAREs syntaxin-1A, synaptobrevin-II (VAMP), and SNAP-25A were expressed separately, and various dye-labeled combinations of the SNAREs were tested for their ability to dock liposomes and induce fusion. Syntaxin and synaptobrevin in opposing membranes were both necessary and sufficient to dock liposomes to supported bilayers and to induce thermally activated fusion. As little as one SNARE interaction was sufficient for liposome docking. Fusion of docked liposomes with the supported bilayer was monitored by the dequenching of soluble fluorophores entrapped within the liposomes. Fusion was stimulated by illumination with laser light, and the fusion probability was enhanced by raising the ambient temperature from 22 to 37 degrees C, suggesting a thermally activated process. Surprisingly, SNAP-25 had little effect on docking efficiency or the probability of thermally induced fusion. Interprotein fluorescence resonance energy transfer experiments suggest the presence of other conformational states of the syntaxin*synaptobrevin interaction in addition to those observed in the crystal structure of the SNARE complex. Furthermore, although SNARE complexes involved in liposome docking preferentially assemble into a parallel configuration, both parallel and antiparallel configurations were observed.

Phosphorylation of proteins in neuron terminals: specificity depends on coincidental signaling

We investigate the role of neuronal coincidental signaling mediated by the second messengers, on phosphorylation of three major proteins of neurosecretory vesicles. Our data show that different combinations of coincidental signaling generate specific pattern of phosphoproteins and not strictly additional effects. This suggests that an added phosphate on a site might 'mask' or 'unmask' the next sites for specific kinases and phosphatases action by inducing conformation change or protein association. We show that a function of vesicles such as the uptake of glutamate is highly regulated by coincidental signaling.

Prolonged retention after aggregation into secretory granules of human R183H-growth hormone (GH), a mutant that causes autosomal dominant GH deficiency type II

Human R183H-GH causes autosomal dominant GH deficiency type II. Because we show here that the mutant hormone is fully bioactive, we have sought to locate an impairment in its progress through the secretory pathway as assessed by pulse chase experiments. Newly synthesized wild-type and R183H-GH were stable when expressed transiently in AtT20 cells, and both formed equivalent amounts of Lubrol-insoluble aggregates within 40 min after synthesis. There was no evidence for intermolecular disulfide bond formation in aggregates of wild-type hormone or the R183H mutant. Both wild-type and R183H-GH were packaged into secretory granules, assessed by the ability of 1 mM BaCl2 to stimulate release and by immunocytochemistry. The mutant differed from wild-type hormone in its retention in the cells after packaging into secretory granules; 50% more R183H-GH than wild-type aggregates were retained in AtT20 cells 120 min after synthesis, and stimulated release of R183H-GH or a mixture of R183H-GH and wild-type that had been retained in the cell was reduced. The longer retention of R183H-GH aggregates indicates that a single point mutation in a protein contained in secretory granules affects the rate of secretory granule release.

Neuropilin-2 is a novel marker expressed in pancreatic islet cells and endocrine pancreatic tumours

Neuropilin-2 (NP-2) is a cell surface transmembrane protein originally characterized as a receptor for the type 3 semaphorins, and more recently for a number of vascular endothelial growth factor (VEGF) isoforms. NP-2 expression has been recently localized to a subset of neuroendocrine cells in the gastrointestinal tract. The aim of this study was to define the expression pattern of NP-2 in normal pancreatic islets and to determine the utility of NP-2 expression as a diagnostic marker of pancreatic endocrine tumours. Paraffin-embedded tissue sections from 30 endocrine pancreatic tumours (EPTs) and from normal pancreas were immunostained with a rabbit polyclonal antibody generated towards NP-2. Nineteen of the tumours were hormonally functional (nine insulinomas, nine gastrinomas, and one glucagonoma). The NP-2 staining pattern was correlated with islet cell hormone expression. In addition, NP-2 expression was evaluated in other normal neuroendocrine tissues and neuroendocrine neoplasms. In normal pancreas, NP-2 stained a distinct subset of islet cells situated primarily at the islet periphery. Double immunohistochemical staining revealed co-localization with glucagon-expressing cells. Moderate to strong NP-2 staining was present in 27 of 30 EPTs. Serial staining of the pancreatic tumours with insulin, gastrin, glucagon, pancreatic polypeptide (PP) or somatostatin did not reveal a distinct pattern of co-localization. NP-2 expression was not detected in neuroendocrine cells outside the gastroenteropancreatic system, or in their corresponding neoplasms, except for focal staining in one bronchial carcinoid tumour. In conclusion, the vast majority of EPTs examined expressed NP-2, suggesting its utility as a diagnostic marker for these tumours. The function of NP-2 in islet cell biology or tumourigenesis remains to be elucidated.

The calcium-binding loops of the tandem C2 domains of synaptotagmin VII cooperatively mediate calcium-dependent oligomerization

Synaptotagmin VII (Syt VII), a proposed regulator for Ca2+-dependent exocytosis, showed a robust Ca2+-dependent oligomerization property via its two C2 domains (Fukuda, M., and Mikoshiba, K. (2001) J. Biol. Chem. 276, 27670-27676), but little is known about its structure or the critical residues directly involved in the oligomerization interface. In this study, site-directed mutagenesis and chimeric analysis between Syt I and Syt VII showed that three Asp residues in Ca2+-binding loop 1 or 3 (Asp-172, Asp-303, and Asp-357) are crucial to robust Ca(2+)-dependent oligomerization. Unlike Syt I, however, the polybasic sequence in the beta4 strands of the C2 structures (so-called "C2 effector domain") is not involved in the Ca2+-dependent oligomerization of Syt VII. The results also showed that the Ca2+-binding loops of the two C2 domains cooperatively mediate Syt VII oligomerization (i.e. the presence of redundant Ca2+-binding site(s)) as well as the importance of Ca2+-dependent oligomerization of Syt VII in Ca2+-regulated secretion. Expression of wild-type tandem C2 domains of Syt VII in PC12 cells inhibited Ca2+-dependent neuropeptide Y release, whereas mutant fragments lacking Ca2+-dependent oligomerization activity had no effect. Finally, rotary-shadowing electron microscopy showed that the Ca2+-dependent oligomer of Syt VII is "a large linear structure," not an irregular aggregate. By contrast, in the absence of Ca2+ Syt VII molecules were observed to form a globular structure. Based on these results, we suggest that the linear Ca2+-dependent oligomer may be aligned at the fusion site between vesicles and plasma membrane and modulate Ca2+-regulated exocytosis by opening or dilating fusion pores.

Synaptotagmin V is targeted to dense-core vesicles that undergo calcium-dependent exocytosis in PC12 cells

Synaptotagmins (Syts) III, V, VI, and X are classified as a subclass of Syt, based on their sequence similarities and biochemical properties (Ibata, K., Fukuda, M., and Mikoshiba, K. (1998) J. Biol. Chem. 273, 12267-12273; Fukuda, M., Kanno, E., and Mikoshiba, K. (1999) J. Biol. Chem. 274, 31421-31427). Although they have been suggested to be involved in vesicular trafficking, as in the role of the Syt I isoform in synaptic vesicle exocytosis, their exact functions remain to be clarified, and even their precise subcellular localization is still a matter of controversy. In this study, we established rat pheochromocytoma (PC12) cell lines that stably express Syts III-, V-, VI-, and X-GFP (green fluorescence protein) fusion proteins, respectively, to determine their precise subcellular localizations. Surprisingly, Syts III-, V-, VI-, and X-GFP proteins were found to be targeted to specific organelles: Syt III-GFP to near the plasma membrane, Syt V-GFP to dense-core vesicles, Syt VI-GFP to endoplasmic reticulum-like structures, and Syt X-GFP to vesicles (other than dense-core vesicles) present in cytoplasm. We showed that Syt V-containing vesicles at the neurites of PC12 cells were processed to exocytosis in a Ca2+-dependent manner. Immunohistochemical analysis further showed that endogenous Syt V was also localized on dense-core vesicles in the mouse brain and specifically expressed in glucagon-positive alpha-cells in mouse pancreatic islets, but not in beta- or delta-cells. Based on these results, we propose that Syt V is a dense-core vesicle-specific Syt isoform that controls a specific type of Ca2+-regulated secretion.

Dual and opposing roles of presynaptic Ca2+ influx for spontaneous GABA release from rat medial preoptic nerve terminals

Calcium influx into the presynaptic nerve terminal is well established as a trigger signal for transmitter release by exocytosis. By studying dissociated preoptic neurons with functional adhering nerve terminals, we here show that presynaptic Ca2+ influx plays dual and opposing roles in the control of spontaneous transmitter release. Thus, application of various Ca2+ channel blockers paradoxically increased the frequency of spontaneous (miniature) inhibitory GABA-mediated postsynaptic currents (mIPSCs). Similar effects on mIPSC frequency were recorded upon washout of Cd2+ or EGTA from the external solution. The results are explained by a model with parallel Ca2+ influx through channels coupled to the exocytotic machinery and through channels coupled to Ca2+-activated K+ channels at a distance from the release site.

NMR measurement of the off rate from the first calcium-binding site of the synaptotagmin I C2A domain

The off rate from the first calcium-binding site of the C2A domain of synaptotagmin I, a putative calcium receptor in neurotransmitter release, has been determined by 15N-nuclear magnetic resonance relaxation dispersion measurements. The exchange rate was obtained by fitting the dependence of the transverse relaxation rates on the interval between 180 degrees pulses in relaxation-compensated CPMG experiments at 3.2 microM calcium concentration. The measured k(ex) is 2.0x10(3) x s(-1). The calcium on rate of 3.5+/-1x10(7) x s(-1), determined from the measured off rate and the dissociation constant (5.3x10(-5) M), is close to the diffusion limit. These results are consistent with the proposed role of synaptotagmin I as a calcium sensor in release, but suggest that additional factors may help to accelerate the diffusion of Ca2+ to the sensor.

Synaptotagmin IX regulates Ca2+-dependent secretion in PC12 cells

Synaptotagmin (Syt) I-deficient phaeochromocytoma (PC12) cell lines show normal Ca(2+)-dependent norepinephrine (NE) release (Shoji-Kasai, Y., Yoshida, A., Sato, K., Hoshino, T., Ogura, A., Kondo, S., Fujimoto, Y., Kuwahara, R., Kato, R., and Takahashi, M. (1992) Science 256, 1821-1823). To identify an alternative Ca(2+) sensor, we searched for other Syt isoforms in Syt I-deficient PC12 cells and identified Syt IX, an isoform closely related to Syt I, as an abundantly expressed dense-core vesicle protein. Here we show that Syt IX is required for the Ca(2+)-dependent release of NE from PC12 cells. Antibodies directed against the C2A domain of either Syt IX or Syt I inhibited Ca(2+)-dependent NE release in permeable PC12 cells indicating that both Syt proteins function in dense-core vesicle exocytosis. Our results support the idea that Syt family proteins that co-reside on secretory vesicles may function cooperatively and redundantly as potential Ca(2+) sensors for exocytosis.

Three-dimensional structure of the synaptotagmin 1 C2B-domain: synaptotagmin 1 as a phospholipid binding machine

Synaptotagmin 1 probably functions as a Ca2+ sensor in neurotransmitter release via its two C2-domains, but no common Ca2+-dependent activity that could underlie a cooperative action between them has been described. The NMR structure of the C2B-domain now reveals a beta sandwich that exhibits striking similarities and differences with the C2A-domain. Whereas the bottom face of the C2B-domain has two additional alpha helices that may be involved in specialized Ca2+-independent functions, the top face binds two Ca2+ ions and is remarkably similar to the C2A-domain. Consistent with these results, but in contrast to previous studies, we find that the C2B-domain binds phospholipids in a Ca2+-dependent manner similarly to the C2A-domain. These results suggest a novel view of synaptotagmin function whereby the two C2-domains cooperate in a common activity, Ca2+-dependent phospholipid binding, to trigger neurotransmitter release.

The development of postganglionic sympathetic neurons: coordinating neuronal differentiation and diversification

The fine-tuned operation of the nervous system is accomplished by a diverse set of neurons which differ in their morphology, biochemistry and, consequently, their functional properties. The accurate interconnection between different neuron populations and their target tissues is the prerequisite for physiologically appropriate information processing. This is exemplified by the regulatory action of the autonomic nervous system in vertebrates to sustain homeostasis under changing physiological demands. For this purpose, the coordination of divergent regulatory responses is required in a multitude of tissues spread over the entire body. To meet this task, diverse neuronal populations interact at different levels. In the sympathetic system. chemical relations between preganglionic and postganglionic neurons appear to differ along the rostrocaudal axis. In addition, postganglionic neurons innervating different target tissues at a segmental level have distinct properties. Differences in their preganglionic innervation and their integrative membrane properties result in diverse activation patterns upon reflex stimulation. Moreover, postganglionic neurons differ in the transmitter molecules they employ to convey information to the target tissues. The segregation of noradrenaline and acetylcholine to different populations of postganglionic sympathetic neurons is well established. A combination of cellular and molecular approaches has begun to uncover how such a complex system may be generated during development. Growth and transcription factors involved in noradrenergic and cholinergic differentiation are characterised. Interestingly, they can also promote the expression of proteins involved in transmitter secretion. As the proteins participating in the vesicle cycle are expressed in many neuron populations, whereas the enzymes of transmitter biosynthesis are restricted to subpopulations of neurons, the findings suggest that early in neuronal development subpopulation-specific and more widely expressed neuronal properties can be commonly induced. Still, many details concerning the signals involved in the induction of the neurotransmitter synthesis and release machinery remain to be worked out. Likewise, the regulatory processes resulting in differences of electrophysiological membrane properties and the specific recognition between pre- and postganglionic neurons have to be determined. Ultimately, this will lead to an understanding at the molecular level of the development of a nervous system with diverse neuronal populations that are specifically interconnected to distinct input neurons and target tissues as required for the performance of a complex regulatory function.

Effects of voltage-sensitive calcium channel blockers on extracellular dopamine levels in rat striatum

Various subtypes of voltage-sensitive calcium channels (VSCCs) support the release of dopamine (DA) in the central nervous system. Using in vivo microdialysis, we investigate the influence of these subtypes of calcium channels on dopaminergic terminals in the rat striatum. L-type (nifedipine-sensitive), N-type (omega-conotoxin GVIA-sensitive), or N- and P/Q-type (omega-conotoxin MVIIC-sensitive) Ca2+ channels were blocked using selective antagonists injected locally, and K+-evoked DA release was measured in freely moving animals. K+ (100 mM) induced a massive increase of basal DA extracellular levels (930%) and was without significant effect on extracellular levels of DA metabolites DOPAC and HVA, and on the serotonin metabolite 5HIAA. Omega-conotoxin GVIA (1 microM) and omega-conotoxin MVIIC (1 microM) significantly reduced the K+-evoked DA release by 55 and 62%, respectively. The simultaneous application of the two conotoxins at the same concentration reduced K+-evoked DA release by 66%. Nifedipine (10 microM) had no significant effect on K-evoked DA release, while neomycin, a nonspecific VSCC blocker, produced a highly significant decrease when applied at 250 and 500 microM (56 and 75%, respectively). The compounds. however, had no effect on basal DA release and on the levels of extracellular DOPAC, HVA, and 5HIAA. These results suggest that under high and persistent conditions of membrane depolarization (15 min, 10 mM K+), striatal DA release is mainly mediated by N-type VSCCs.

Neuroendocrine cells along the digestive tract express neuropilin-2

Neuropilin-2 (np-2) is a receptor for semaphorin-3F (sema-3F) and semaphorin-3C (sema-3C). These semaphorins repel tips of growing axons that express np-2. In addition, np-2 functions as a receptor for heparin binding forms of the angiogenic factor vascular endothelial growth factor (VEGF) such as VEGF145 and VEGF165. We report that np-2 is strongly expressed in neuroendocrine cells located all along the human digestive tract. Confocal fluorescent microscopy revealed that np-2 is concentrated in vesicle-like structures located near the nucleus at the basolateral side of these cells. In the colon, the np-2-expressing subpopulation of neuroendocrine cell is almost identical with the serotonin-producing subpopulation of neuroendocrine cells. Gastrointestinal carcinoid tumors are digestive tract tumors that develop from neuroendocrine cells. Interestingly, most of the carcinoid tumors derived from the colon and the appendix did not contain np-2-producing cells. However, some carcinoid tumors derived from the small intestine and stomach did express low levels of np-2 in isolated foci of cells. By contrast, strong serotonin and chromogranin-A expression was observed in all of the carcinoid tumors that were examined. These results suggest that loss of np-2 expression may accompany tumor progression in carcinoid tumors.

Synaptotagmin I functions as a calcium regulator of release probability

In all synapses, Ca2+ triggers neurotransmitter release to initiate signal transmission. Ca2+ presumably acts by activating synaptic Ca2+ sensors, but the nature of these sensors--which are the gatekeepers to neurotransmission--remains unclear. One of the candidate Ca2+ sensors in release is the synaptic Ca2+-binding protein synaptotagmin I. Here we have studied a point mutation in synaptotagmin I that causes a twofold decrease in overall Ca2+ affinity without inducing structural or conformational changes. When introduced by homologous recombination into the endogenous synaptotagmin I gene in mice, this point mutation decreases the Ca2+ sensitivity of neurotransmitter release twofold, but does not alter spontaneous release or the size of the readily releasable pool of neurotransmitters. Therefore, Ca2+ binding to synaptotagmin I participates in triggering neurotransmitter release at the synapse.

alpha-Latrotoxin releases calcium in frog motor nerve terminals

alpha-Latrotoxin (alpha-LTX) is a neurotoxin that accelerates spontaneous exocytosis independently of extracellular Ca(2+). Although alpha-LTX increases spontaneous transmitter release at synapses, the mechanism is unknown. We tested the hypothesis that alpha-LTX causes transmitter release by mobilizing intracellular Ca(2+) in frog motor nerve terminals. Transmitter release was measured electrophysiologically and with the vesicle marker FM1-43; presynaptic ion concentration dynamics were measured with fluorescent ion-imaging techniques. We report that alpha-LTX increases transmitter release after release of a physiologically relevant concentration of intracellular Ca(2+). Neither the blockade of Ca(2+) release nor the depletion of Ca(2+) from endoplasmic reticulum affected Ca(2+) signals produced by alpha-LTX. The Ca(2+) source is likely to be mitochondria, because the effects on Ca(2+) mobilization of CCCP (which depletes mitochondrial Ca(2+)) and of alpha-LTX are mutually occlusive. The release of mitochondrial Ca(2+) is partially attributable to an increase in intracellular Na(+), suggesting that the mitochondrial Na(+)/Ca(2+) exchanger is activated. Effects of alpha-LTX were not blocked when Ca(2+) increases were reduced greatly in saline lacking both Na(+) and Ca(2+) and by application of intracellular Ca(2+) chelators. Therefore, although increases in intracellular Ca(2+) may facilitate the effects of alpha-LTX on transmitter release, these increases do not appear to be necessary. The results show that investigations of Ca(2+)-independent alpha-LTX mechanisms or uses of alpha-LTX to probe exocytosis mechanisms would be complicated by the release of intracellular Ca(2+), which itself can trigger exocytosis.

Accumulation of synaptosomal-associated protein of 25 kDa (SNAP-25) and other proteins associated with the secretory pathway in GH4C1 cells upon treatment with estradiol, insulin, and epidermal growth factor

Treatment of rat pituitary GH4C1 cells with estradiol, insulin, and epidermal growth factor induces secretory granule accumulation, PRL storage, and stabilization of ICA512, a membrane protein associated with secretory granules. In these investigations we found that the same treatment induced accumulation over 2-fold of other proteins in the secretory pathway, including synaptosomal-associated protein of 25 kDa (SNAP-25), synaptotagmin III, synaptobrevin, synaptophysin, and cyclophilin B, and did not affect accumulation of others, including synaptotagmin I, calnexin, and glucose-regulated protein 94. The induction of proteins was not a coordinate event, because epidermal growth factor alone maximally stimulated SNAP-25 accumulation, but not that of synaptotagmin III. Induction of SNAP-25 accumulation occurred without an increase in its synthesis, and induction of cyclophilin B occurred without an increase in its messenger RNA accumulation, suggesting that accumulation may be caused by stabilization of the proteins. SNAP-25 immunofluorescence was located in the cytoplasm and on the plasma membrane and sometimes was heavily concentrated in protrusions from the cell surface, especially in hormone-treated cells. Frequenin immunofluorescence was also sometimes concentrated in intense patches, but did not colocalize with SNAP-25. Growth hormone and prolactin immunofluorescence was not found in the protrusions and sometimes did not colocalize with each other when they were present in the same cell. Hormone treatment of GH4C1 cells therefore induces accumulation of specific proteins in all parts of the secretory pathway and causes morphological changes in addition to accumulating secretory granules.

Synaptotagmins in membrane traffic: which vesicles do the tagmins tag?

The aim of this review is to give a broad picture of what is actually known about the synaptotagmin family. Synaptotagmin I is an abundant synaptic vesicle and secretory granule protein in neurons and endocrine cells which plays a key role in Ca(2+)-induced exocytosis. It belongs to the large family of C2 domain-proteins as it contains two internal repeats that have homology to the C2 domain of protein kinase C. Eleven synaptotagmin genes have been described in rat and mouse. Except for synaptotagmin I, and by analogy synaptotagmin II, the functions of these proteins are unknown. In this review we focus on data obtained on the various isoforms without exhaustively discussing the role of synaptotagmin I in neurotransmission. Numerous in vitro interactions of synaptotagmin I with key components of the exocytosis-endocytosis machinery have been reported. Additional data concerning the other synaptotagmins are now becoming available and are reviewed here. Only interactions which have been described for several synaptotagmins, are mentioned. It is unlikely that a single isoform displays all of these potential interactions in vivo and probably the subcellular distribution of the protein will favor some of them and preclude others. Therefore, to discuss the putative role of the various synaptotagmins we have examined in detail published data concerning their localization.